1. Field of the Invention
The present invention relates generally to field emission electron sources, and more particularly, to a field emission electron gun wherein the effective brightness over a wide range of potentials is greatly improved by utilizing a selective scaling method.
2. Description of the Prior Art
A field emission electron source offers the highest brightness of all electron sources for a given potential. One example of such a source is a field emission electron gun. Field emission electron guns are widely used in electron microscopes and other electron optical systems. FIG. 1 is a cross-sectional view of a portion of a conventional field emission electron gun 10. Field emission electron guns are well known in the art and thus, only that portion of conventional field emission electron gun 10 which is needed to understand the problems with conventional electron guns is shown in FIG. 1. A more detailed description of a conventional prior art electron gun is shown, for example, in U.S. Pat. No. 3,931,519, which patent is hereby incorporated by reference. The electron gun 10 includes a field emission tip 12, a first planar electrode 14 having a thickness t.sub.1 and a second planar electrode 16 having a thickness t.sub.2. The distance between the tip 12 and first electrode 14 is represented by Z.sub.o and the distance between the electrodes is represented by S. Electrode 14 is separated from electrode 16 by dielectric layer 18. The combination of the first electrode, dielectric layer and second electrode is also referred to as a dual-electrode immersion lens. A well defined and accurately aligned circular hole is present at the center of each electrode and the dielectric layer to allow for the passage of electrons. The diameter of the circular hole in electrode 14 is indicated by D.sub.1 and the diameter of the circular hole in electrode 16 is indicated by D.sub.2. To provide for proper lens action, the diameter of the circular hole in the dielectric layer D.sub.3 is larger than the bore diameter D.sub.1 of the first electrode and the bore diameter D.sub.2 of the second electrode.
In operation of the conventional field emission electron gun 10, an extraction potential, V.sub.1, in the order of one to a few thousand volts is applied between the tip 12 and the first electrode 14 to cause field emission to take place and an additional potential, V.sub.2, applied between the two electrodes accelerates the electrons to their final potential (V.sub.1 +V.sub.2) and also creates a lens action forming a real or virtual image of the electron beam emitted from the tip.
The performance of a field emission electron gun measured in brightness can be improved by reducing the lens aberrations. The three main aberrations are diffraction, spherical and chromatic aberration. The last two are related to the geometry of the lens and can, therefore, be reduced by scaling down the physical dimensions of the lens. Thus, in designing a field emission electron gun, it is desirable to optimize the dimensions of the lens and the distance between the tip and first electrode to achieve the lowest possible values for the lens aberrations. However, two practical constraints need to be considered before miniaturizing the lens dimensions. First, the bore diameter of the electrode cannot be reduced to much less than 1 .mu.m in order to be compatible with the tolerance of the microfabrication techniques presently available. Second, the electric field between the electrodes must be restricted to a value not exceeding 10.sup.4 V/mm to avoid breakdown.
In a conventional field emission gun, the optimum lens dimensions are set by the high degree of accuracy required, especially in the definition of the circular hole, tip position and the tolerances of the mechanical fabrication processes employed. The typical dimensions and computed aberration coefficients (referenced to the object space) of the optimized conventional field emission gun 10 for operation at 200 V, 1 kV and 10 kV are shown in FIG. 2. It should be pointed out that, apart from the spacing between the electrodes S which varies with the value of V.sub.2 applied, the values of the electrode bore diameters D.sub.1 and D.sub.2, electrode thicknesses t.sub.1 and t.sub.2 and tip to first electrode spacing Z.sub.o are typical for guns at other potentials. As can be seen from FIG. 2 these values are generally limited by the considerations discussed above to the millimeter range. In addition, the values of the coefficients of spherical aberration (C.sub.so) and chromatic aberration (C.sub.co) are also in the millimeter range.
The distance between the tip and first electrode can be reduced to the micron range by the use of a scanning tunneling microscope (STM) which also makes the use of a lens with electrode diameter in the micron range (microlens) feasible. This lead to the development of a STM controlled field emission tip in conjunction with a microlens to form a STM-aligned field emission (SAFE) microsource which can provide a significant improvement in emission stability and brightness.
The basic concept of the SAFE microsource is to utilize the STM feedback principle for precision x, y and z alignment of a field emission tip to a microlens to form a microsource which can be used by itself or in conjunction with another microlens to form a focused probe of electrons. The microlenses can be made using many of the standard integrated circuit fabrication techniques on silicon or other substrates with dimensions reduced to micrometer scale. As the lens aberrations generally scale with lens dimensions, such lenses can be designed to have aberrations which are 2 to 3 orders of magnitude less than a conventional field emission source. The reduction in lens aberrations results in 2 to 3 orders of magnitude improvement in brightness compared to conventional field emission sources at the same potential.
For a field emission electron gun consisting of a tip and a two-electrode immersion lens, conventional scaling requires that the lens dimensions (D.sub.1, D.sub.2, S) and the tip spacing Z.sub.o be uniformly reduced by the same scaling factor, k, from a conventional design to achieve a reduction of approximately the same factor k for the aberrations as shown by the SAFE microsource. However, this increases the electric field between the electrodes by a factor equal to the inverse of k, and for a small k value, the field can exceed the voltage breakdown threshold of 10.sup.4 V/mm rendering the dimensions of the scaled emission gun impractical. Since conventional lenses are designed fairly close to this field strength limit, very little room is left for scaling. This limitation also applies to the immersion lens in the SAFE microsource and prevents the SAFE microsource from operating and increasing brightness over a wide range of potentials. Thus, there is a need to develop a scaling method which will allow a field emission electron gun to operate and maintain low aberrations over a wide range of potentials and be compatible with voltage breakdown and electrode bore diameter constraints.